1. Field of the Invention
The present invention relates to a current-differential relaying method and system for protecting a transformer more correctly by calculating and estimating an exciting current including a core-loss current and a magnetizing current, and, in particular, to a current-differential relaying method and system which can protect the transformer correctly irrespective of the level of remanent flux.
2. Description of the Prior Art
As in the case of a generator or a power transmission line, differential relays have been widely used for protecting a transformer from internal faults. Herein the term ‘differential current relay’ indicates the method of calculating the difference of currents flowing through both terminals of the transformer (hereinafter, referred to as “differential current”), and detecting an internal fault when its value exceeds a certain predetermined value.
FIG. 1 shows a configuration of a relay system for transformer protection according to a conventional current differential relay. As shown in FIG. 1, the conventional current differential relay system comprises a transformer 40 connecting both terminals 10, 70 in the power system, a primary current measurement unit 30 of the transformer, a secondary current measurement unit 50 of the transformer, a transformer protection controller 80 for driving circuit breakers 20, 60 for protecting the transformer by using a primary current and a secondary current measured at said current measurement units.
However, when the transformer is energized, a large current flows through the primary winding and a large differential current occurs, which is referred to as a “magnetic inrush (or inrush)” and which results in a problem that the differential current occurs even when internal faults are not present. In order to prevent the transformer protection relay from mal-operating, the transformer protection relay must discriminate correctly magnetic inrush or over-excitation from an internal fault.
Generally, because the primary current includes harmonic components, a second harmonic restraining current-differential relaying method is used in order to discriminate an internal fault from magnetic inrush, in which the differential current is an operating current for relay operation and the second harmonic component is a restraining or blocking current. Furthermore, in order to discriminate between an internal fault and over-excitation, a fifth harmonic restraining current-differential relaying method may be used in which the fifth harmonic component is a restraining or blocking current. However, these methods have the limit of application due to the difficulty of accurate discrimination between an internal fault and magnetic inrush, in that the magnitude of each harmonic component may be varied depending on whether or not there is the remanent flux in the iron core or depending on the magnitude of the remanent flux, and in that, when a power condition and a condition such as quality of transformer core material are changed, a large amount of the second harmonic component may be included even in the case of an internal fault. Furthermore, the harmonic components exist in a transient period after an internal fault occurs. Therefore, it needs a substantial amount of time until the harmonic components become zero and thus rapid fault detection is difficult. And, the restraining or blocking methods using these harmonic components may prevent maloperation to some extent in the case of magnetic inrush or over-excitation, but they cannot prevent maloperation in the case that the differential current has a small amount of harmonic components.
To solve this problem, a relaying method was proposed using magnetic flux derived from the primary voltage. This method employed the principle that the magnetizing current and the flux comply with the magnetization curve during magnetic inrush, while the flux is proportional to the magnetizing current and the ratio of the change in the flux to the change in the magnetizing current becomes small during an internal fault. However, there is a problem that, if there is the remanent flux, the errors occur because the locus of the magnetizing current versus flux deviates from the magnitization curve.
Accordingly, a method was proposed using the ratio of the change in the flux to the change in the magnetizing current, i.e. the slope of the magnetization curve. If the slope is large, a counter decreases; if the slope is small, the counter increases. When the counter exceeds a preset value, a trip signal is activated. However, there is a problem that this method cannot apply to the over-excitation and the magnetic inrush on a loaded transformer, because the primary current and the differential current are not equal to each other.
Furthermore, in order to provide a method which can be also applied to the case that there is the remanent flux, a method was proposed using the region bounded with the lower and upper limits rather than the magnetization curve. The former is the flux on the curve minus the maximum remanent flux and the latter the flux on the curve plus the maximum remanent flux. However, according to this method, as the maximum remanent flux is 80% of the saturation-point flux, the bounded region becomes very wide and thus includes the region for an internal fault and the operating time for an internal fault is inevitably delayed.
In order to the shortcomings of these current differential relays, transformer model-based protection methods were proposed. These methods may perform a very rapid calculation without calculating the phasors of voltage and current. However, they have the limit of application in that a large amount of data should be measured because they need the voltages as well as the currents of both terminals of the transformer.
Hereinafter, a conventional compensated-current differential relay suitable for protection of power transformer is described. FIG. 2 shows a three-phase Y-Y transformer 40, and FIG. 3 shows the per phase equivalent circuit of the transformer 40. The nomenclature used in the figures is as follows:
v1A, v1B, v1C, v2A, v2B and v2C: primary and secondary voltages of each phase;
i1A, i1B, i1C, i2A, i2B and i2C: primary and secondary currents of each phase;
v1 and v2: primary and secondary voltages;
e1 and e2: primary and secondary induced voltages;
i1 and i2: primary and secondary currents;
R1 and R2: primary and secondary winding resistances;
L11 and L12 primary and secondary leakage inductances;
Rc: core-loss resistance;
Lm: magnetizing inductance;
N1 and N2: numbers of primary and secondary windings;
ie: exciting current;
ic: core-loss current; and
im: magnetizing current
A conventional differential relay derives the magnitude of the differential current using:
                              I          d                =                                                                      I                ρ                            1                        -                          a              ⁢                                                I                  ρ                                2                                                                                  (        1        )            
where, Iρ1 and Iρ2 are the phasors of the fundamental component of the primary and secondary currents, respectively, and a=N2/N1. And the magnitude of the fundamental component of the restraining current Ir for restraining or blocking the relay operation is obtained by:
                              I          r                =                                                                                          I                  ρ                                1                            +                              a                ⁢                                                      I                    ρ                                    2                                                                          2                                    (        2        )            
And the characteristic of the relay is given by:Id≧Ioffset+KIr, Ioffset=15A  (3)
where K represents the sensitivity of relay and may be set arbitrarily. The below data is based on 0.3 of the sensitivity of the relay. According to this conventional current differential relay, as seen in equation (1), the differential current does not include the exciting current ie(t). Thus, as ie(t) becomes significant during magnetic inrush or over-excitation, the differential current,
            I      d        =                                              I            ρ                    1                -                  a          ⁢                                    I              ρ                        2                                      ,may exceed the restraining threshold and the conventional relay will mal-operate.